1. Field of the Invention
The present invention relates to an electric resistance welding, and more particularly, to a method and apparatus for resistance welding.
2. Description of the Related Art
In an electric resistance welding such as a spot welding, workpieces are forced together between a pair of upper and lower opposing electrodes and are welded to each other by applying a current therethrough for a predetermined time. In this widely used electric resistance welding method, electrode tips 1 and 2 are fixed directly to electrode holders 5 and 6, respectively, which are fixed to arms (also called horns) 7 and 8, respectively, or are fixed to the electrode holders by shanks (also called sleeves) 3 and 4, respectively, as shown in FIG. 2. The electrode assembly having such a construction has a sufficient electrode tip life if the workpieces are of a usual cold-rolled steel sheet, but where workpieces of a coated steel sheet such as a galvanized steel sheet are welded by electric resistance welding, as the number of welds increases, the electrode tips made of a material containing copper are alloyed by the coated metal on the steel sheets, resulting in wear and enlargement of the electrode tip face (contact with the steel sheet), whereby the current density is lowered and a specified nugget cannot be formed. Therefore, the productivity is unavoidably lowered because the time (number of welds) at which the electrode tip must be dressed or replaced is shortened.
Means for solving these problems are known as disclosed, for example, in Japanese Unexamined Patent Publication No. Sho 61-293679 and Japanese Unexamined Utility Model Publication No. Sho 61-67978. The welding machines disclosed in these Publications have an electrode forcing axis, electrode rotating axes disposed perpendicularly to the electrode forcing axis, and a plurality of electrode tips buried around the electrode rotating axis or an annular electrode disposed around the electrode rotating axis, which is intermittently rotated for welding. These machines are disadvantageous, however, in that they are large in scale and the workpieces that can be welded thereby are limited due to the structure thereof.